Making a Soundproof Door

I am a fairly skilled carpenter, and am just starting to build a house in southern Mexico (where I live). I want to make the bathroom as quiet (on the outside) as possible. The walls are going to be a panel of concrete and pulyurathane combined (5" thick) and so I think the door is the weak spot. I am wondering if I make a door with the two faces from thick (1/2") plywood or MDF, brace them internally on a system of ribs that only are attached to one face interspersed with ribs on the other face (but not touching except at the rim) would give a substanially noise-reduced door. Any ideas out there?